The present invention relates to an improved method for carrying out specific binding assay tests. More particularly the invention relates to an improved method to be performed in an automatic manner for carrying out specific binding assay tests wherein liquid and solid phases are present.
As known, specific binding assays are based on the principle of monitoring specific binding reactions, in which the extent of binding is a function of the amount of unknown ligand present, by means of a labelled component. Among the known methods the following specific binding assay techniques can be mentioned: Radioimmunoassay (RIA), metalloimmunoassay (MIA), free radical assay technique (FRAT), hemaglutination inhibition (HI), enzyme multiplied immunoassay technique (EMIT), fluorescence immunoassay (FIA) and luminescence immunoassay (LIA). In some of these techniques (RIA, MIA, FIA, LIA) the mixture comprising the unlabelled ligand, labelled ligand and antibody is allowed to reach an equilibrium and the antibody-bound ligand is separated from the free ligand. In the radioimmunoassay, the ligand or the antibody is labelled with a radioactive isotope, whereas in the metalloimmunoassay the ligand is labelled with a metal-containing reagent which contains also a suitable functional group by means of which one can attach the metal reagent to the hapten desired to be assayed. A full description of the latter is given in our previous U.S. Pat. No. 4,205,952. In FIA the label is a fluorescent compound and in LIA the label is chemiluminescent or bioluminescent agent.
The operation of separating the free fraction from the bound one, is of great importance and its accuracy determines the sensitivity and precision of the entire specific binding assay technique. In selecting and assessing a separation operation, it is useful to consider the criteria that should be fulfilled in order to obtain the desired result. The following major requirements of an ideal separation can be mentioned:
(i) It should completely separate bound and free fractions, with a wide margin for error in the conditions used for separation; PA1 (ii) It should not interfere with the primary antigen-antibody binding reaction; PA1 (iii) It should be simple, easy and rapid to use; PA1 (iv) It should be inexpensive and use reagents and equipment that are readily available; PA1 (v) It should not be affected by plasma or serum; PA1 (vi) All manipulations should be performed in a single tube; PA1 (vii) It should be suitable for automation; PA1 (viii) It should be applicable to a wide range of antigens; PA1 (ix) The manipulative steps in radioimmunoassays should be designed so that they ensure maximum safety from radiation hazards resulting from handling the radioactive reaction system. PA1 (a) arranging the mixing reservoirs in a rack specially designed to hold a number of said mixing reservoirs with the mixer-separators; PA1 (b) introducing the reagents and analytes into said mixing-reservoirs; PA1 (c) capping said mixing reservoirs with said mixer-separators; PA1 (d) allowing the reagents and analytes to incubate for a required period of time in above mixing reservoirs capped with the mixer-separators; PA1 (e) placing the rack carrying the above separator devices with the incubated reagents and analytes into a press-device specially designed to perform at a controlled rate a downward movement whereby the mixer separators are pushed downwards into the mixing reservoirs at a chosen rate and for a preselected distance to complete the desired mass-transport and separation operation; PA1 (f) operating the downward movement of said press-device at the preselected rate and distance; PA1 (g) removing the rack upon the disengagement of the press-device; and PA1 (h) placing the separator devices into the desired analytical instrument for a quantitative or qualitative measurement of either one, or both, of the separated phases as required.
A critical review of the variety of methods available and the extent to which each method approaches all or any of the ideal requirements mentioned above is beyond the scope of this specification. The most widely used techniques mentioned by the prior art are adsorption methods (charcoal, silicates), fractional precipitation methods (ammonium sulphate, ethanol, dioxan, polyethylene glycol), double antibody methods and solid phase methods (immunoadsorbants), all of which end up with a system of suspended particles in a liquid medium. The selection of any particular technique is determined by consideration of many interrelated factors such as solubility of compound, characteristics of antiserum, fraction to be counted, degree of non-specific binding, type of radioisotope. However one feature which is common to all the above methods is the need for a centrifugation step to effect aggregation of the suspended solid particles followed by a decantation (or suction) step to physically separate the solid and liquid phases.
In a previous patent application (Ser. No. 124,691 assigned to Technion Research & Development Foundation Ltd., it has been disclosed a method to be used in specific binding assay, wherein the separation of the bound fraction from the free fraction is carried out by solvent extraction technique, using organic solvents as extractants. In another prior patent application (Ser. No. 212,806 in the names of M. Cais, M. Shimoni and Technion Research & Development Foundation Ltd. a newly designed device designated as "LIDEX" for carrying out said technique of solvent extraction has been described. According to said invention, the "LIDEX" device consists of a mixing-reservoir (A) into which is fitted snugly a mixer-separator (B), having a channel in the vertical axis of the mixer-separator. The two substantially immiscible liquid solutions are introduced into the mixing reservoir, the phases are thoroughly mixed by moving the mixer-separator (B) in and out the mixing-reservoir (A). After the spontaneous separation into an upper and lower phase, the upper phase is removed by pushing in the mixer-separator, said upper phase being accumulated in a collecting container (E).
In another prior patent application (Ser. No. 270,411) in the names as in the previous one, mass transport separations for various purposes including specific binding assays, to be carried out through selective barriers, have been disclosed. The invention discloses a new "Lidex" device similar to that described in the U.S. patent application Ser. No. 212,806 a barrier being located in the mixer-separator. The resistance raised to the flow of the liquid phase through the membrane into the mixer-separator will generally cause a penetration of the fluid around the sealing element located on the mixer-separator, during the downward gliding of the mixer-separator, which of course will completely interfere the assay. In order to remedy said difficiency, the device is provided with means for accumulating of gas pocket such as one or more horizontal, vertical or spiral grooves on the mixer separator in which the air located therein, will decrease the pressure exerted on the barrier so that said penetration of the fluid around the sealing element is avoided. The invention was found to give excellent results in various systems and for different membranes and types of solvents and/or precipitates.
One of the main requirements encountered in immunoassay, is the reproducibility of the results with a minimal deviation between two duplicates, which implies a complete standardization of the procedure with minimal handling and manual working without depending on extraneous factors. An example of such an extraneous factor is the extent of mixing of the phases in the assay. Another extraneous factor is the rate of separating the desired phase which has to be subsequently analysed.
In the Lidex device without a barrier, used in immunoassay, the test requires a vigorous and thorough agitation to enable a complete mass transfer and an accurate separation between the two liquid phases. As will be realized, the agitation obtained by manual moving the mixer-separator (B) in and out the mixing-reservoir (A), can not be interpreted quantitatively, being actually of a subjective character according to the technician performing the immunoassay. The problem is even more complicated in the case of Lidex device with a membrane, when any different system may necessitate a specific membrane and/or solvent and accordingly will require diverse extent of mixing and/or different rate of separation. This of course will be very difficult or even impossible by a manual handling, particularly for immunoassay, when high accuracy with the closest possible reproducible results are required. Even a technician versed in the art of immunoassay, could hardly assure that a complete mass transfer was accomplished after a certain agitation period. On the other hand, a prolonged agitation might interfere with easy phase separation, when two liquids are involved, or causes damage to the membrane, when precipitates are present.